Visual activity patterns in the developing brain shape the formation of circuits which process that activity. While this is well known, the mechanisms by which this activity exerts these effects remains a critical problem in the field of visual system development. One possibility is that electrical activity in neurons triggers the synthesis of new genes and their protein products. These activity-dependent proteins could then modify the growth of neurons and the formation of synaptic connections. In this proposal, a series of experiments are proposed to test the hypothesis that activity can modify the structure of neurons in the visual system of the albino frog Xenopus laevis. This experimental system permits the in vivo observation of the growth of single neurons in the intact brain of the anesthetized animal. By combining this approach with virus-mediated heterologous expression, and electrophysiological recordings, it will be possible to test whether the expression of two recently identified activity-regulated genes have an impact on tectal development. Preliminary data indicate that these proteins will indeed produce robust changes.